With the burgeoning of electronic technologies, the demand for a wider variety of communications services has been steadily rising. New technologies have been continually developing to meet these demands including lasers, fiber optics, microprocessors, and very large scale integrated circuits. While many alternative approaches for realizing the new communications services are available, digital data transmission seems to be the one best equipped for future progress and, accordingly, digital transmission is currently receiving the most attention.
In terms of the services to be offered, it is expected that, in addition to standard telephone services, a host of low, medium and wide bandwidth services would be introduced. This would include electronic mail, facsimile, high-fidelity audio, computerized data base searches (e.g., want ads, encyclopedia, travel arrangements, etc.), remote shopping, electronic banking and home computer networks. Business users would also use electronic blackboards, teleconference facilities, word processors, and large computer communication networks.
In one copending application, entitled "A Self Routing Switching Network", filed concurrently herewith, S. Knauer and I disclosed a wideband self-routing switch for simultaneously switching a plurality of incoming signals. We described specific embodiments that switch signal packets with the aid of sorting networks and have shown that a wideband switch, which can grow modularly, is very useful.
In a second copending application, entitled "A Wideband Digital Switching Network", also filed concurrently herewith, we enhanced the wideband switch and disclosed a switching network that can offer one-to-one, one-to-many, and many-to-many types of communications, making it feasible to offer the wideband services described above by treating each such switching network as a self contained central office or a "star". What remains, then, is to interconnect the central offices or the stars in the most efficient manner.
A number of approaches are employed in the prior art to manage the communication interconnections between stars. One approach uses high capacity lines to interconnect stars, but this is inefficient because most of the time some capacity is idle. The inefficiency is reduced in a variation on this approach which switches the information onto a small number of lines, taking advantage of the fact that not all of the lines are active simultaneously. This is the classic concentration function. It improves the utilization of the communication paths at the cost of possible blocking of signals.
To reduce blocking of particularly vital signals, a third approach employs a computer to sequentially examine incoming lines, store the applied signal packets in a prioritized queue in the computer's memory and transmit the highest priority packets to its outgoing lines, as they become available. The difficulty with this approach is that a statistical multiplexer must scan the input, properly queue the data, and manage the outgoing lines in essentially a simultaneous manner. This becomes more and more difficult as the data rate increases or the number of incoming and outgoing lines increases. With present technology, the sequential nature of the computer approach cannot meet the demands imposed by the parallelism and consequent high "throughput" rates of the switching network we disclosed.